David Cameron has previously said giving prisoners the vote makes him "physically ill" but is now reportedly planning to abide by a European Court of Human Rights ruling.

If the bill goes ahead, the PM faces yet another showdown with Eurosceptic Tory MPs, who want a referendum on the UK's involvement with the EU.

Government sources played down the speculation, insisting that no decision had yet been taken.

But backbench Tory MPs were quick to jump on the attack.

Nick de Bois, secretary of the influential 1922 committee, posted on Twitter: "Sitting working with 5 other Cons MPs - if reports of prisoner voting rights are accurate then that's 6 MPs who won't vote for it."